1. Field of the Invention
This invention relates to an ignition system for an internal combustion engine.
2. Description of the Related Art
A known ignition system of an internal combustion engine, for example, cuts off a primary current after applying the primary current to a primary winding. This generates a high voltage that is supplied to a spark plug at a secondary winding. The ignition system disclosed in JP-A-2001-193622 energizes the primary winding again at a timing corresponding to an operating state (for example, engine speed and load) of the internal combustion engine after cutting off the primary current, thus cutting off electric power supplied to the spark plug from the secondary winding. This ignition system prevents electric power from being excessively supplied to the spark plug, and thus enhances the durability of the spark plug.
3. Problems to be Solved by the Invention
The ignition system disclosed in JP-A-2001-193622 does not take into consideration that electric power consumed by the time spark discharge occurs depends on the physical state of the spark plug. Hence, if electric power consumed by the time spark discharge occurs is increased, electric power supplied to the spark plug during the spark discharge is excessively reduced. As a result, there may be a shortage of electric power required for ignition by spark discharge. For example, if a spark gap in the spark plug increases due to electrode wear, a higher voltage will be required to generate spark discharge, resulting in greater loss of electric power caused by a step-up in voltage between the electrodes. In addition, if carbon deposits (fouling or smoking dirt) are present due to carbon adhesion in the spark plug, the insulation resistance between electrodes will decrease, resulting in yet greater loss of electric power due to current leakage. As described above, the increased spark gap and the presence of carbon deposits will increase electric power consumed by the time spark discharge occurs.